A major use of sodium and potassium alkali metal salts of 2-ethylhexanoic acid is in the conversion of higher molecular weight organic acids to the corresponding salt of such acid by metathesis. For example, because sodium and potassium 2-ethylhexanoate generally are highly soluble in common organic solvents such as methanol, ethanol, acetone, and the like, it is a convenient process to add such salt to an organic solutiion of a higher molecular weight organic acid, thereby converting the higher molecular weight organic acid to the corresponding salt. The salt of such higher molecular weight acid so formed normally crystallizes out of the organic solution, leaving any unused reactants and by-products still in solution. Such process for salt formation is of particular importance in the case of hard to purify organic acids such as penicillanic acid and cephalosporanic acid antibiotics. U.S. Pat. No. 3,928,592, for instance, describes the reaction of sodium 2-ethylhexanoate with the hard to purify 7-(D-2-formyloxy-2-phenylacetamido)-3- (1-methyl-1H-tetrazole-5-ylthiomethyl)-3-cephem-4-carboxylic acid to provide the corresponding sodium salt of such acid in the form of a highly crystalline pure solid.
In order to effectively utilize either sodium or potassium 2-ethylhexanoate in the aforementioned metathetic reactions, it is important that such salts be of a high purity themselves, and of an easily handled character. Heretofore, the preparation of such alkali metal salts has been accomplished by simply titrating an aqueous solution of the alkanoic acid with the appropriate alkali metal hydroxide, and then adding to the aqueous solution a suitable organic anti-solvent in order to force the alkanoic acid salt out of solution and into a solid form. Such process suffers from various drawbacks, including high cost due to the use of organic anti-solvents and safety hazards resulting from working with large volumes of such solvents. Even more serious drawbacks of such process, however, include the nature of the sodium and potassium 2-ethylhexanoate so formed, which salts typically are impure and of a poor physical character in that the precipitated solid normally is tacky, of undesirable crystalline form, hard to dry, and generally difficult to manage, especially on a commercial scale, due to its extreme hygroscopic properties.
Other processes for preparing the sodium and potassium salts of 2-ethylhexanoic acid include freeze drying an aqueous solution of such salt. This method is commercially unfeasible, however, since an aqueous solution containing about fifty percent by weight of sodium or potassium 2-ethylhexanoate fails to remain frozen under normal freeze-drying conditions. Simple tray-drying of such aqueous solutions is commercially unfeasible due to the extreme length of time required to effect dehydration, and more importantly because such process fails to provide a product of acceptable crystalline quality.
A process for preparing sodium and potassium 2-ethylhexanoate having improved purity and crystalline character, in addition to having improved handleability, and thus obviating the problems of the prior art processes, has now been invented and is the subject of this disclosure.